The present invention relates generally to radio base stations used in wireless telecommunication systems. In particular, it pertains to a small low-heat dissipating radio base station that is especially suitable for indoor applications.
The explosive growth in the wireless telecommunications industry has fueled the demand for a vast array of telecommunication services that are either currently being offered or planned for implementation. These services include traditional analog and digital cellular, and Personal Communication Services (PCS) that include voice, paging, data, and fax capabilities. By many indications, these services will become increasingly popular in the coming years leading, in all likelihood, to expectations of higher levels of service. For example, the ability to access these services from more and more locations becomes an increasingly important issue. Furthermore, the search for more revenue has service providers increasingly interested in being able to provide access to their services in areas that were previously inaccessible. For example, it would be desirable to provide coverage in previously untapped regions such as large indoor areas due to the lack of coverage from conventional outdoor equipment. Such regions may include hotel lobbies, subway stations, restaurants, convention and entertainment centers, office buildings and other situations where localized wireless coverage is required or where subscriber concentrations and call volumes are high.
In a cellular telecommunication system, a mobile switching center (MSC) is linked to a plurality of base stations that are geographically dispersed to form the area of coverage for the system. The radio base stations (RBS) are designated to cover specified areas, known as cells, in which two way radio communication can then take place between the mobile station (MS) and RBS in the coverage area. Although originally conceived for outdoor environments, this idea can be adapted to provide indoor coverage by installing radio base stations in these indoor areas. These RBSs are typically smaller than the outdoor variety and provide coverage by creating micro cells over the region.
Although performance of these indoor systems have been adequate, there are some drawbacks with the design of existing RBSs. For example, it is desirable to reduce the size of the indoor base stations further so that they would be much more unobtrusive and simpler to mount. Very small RBSs, in addition to enhancing aesthetics, allows for simplified mounting and reduces installation costs. For example, very small RBSs would be able to be mounted on existing structures, support beams, or mounted on a wall as opposed to requiring dedicated support structures or special mounting arrangements. One major factor that has inhibited reduction of RBSs to very small sizes has been the relatively large heat dissipating devices required for proper operation.
FIG. 1 shows a perspective view of a prior art Ericsson RBS 884 Micro Radio Base Station 10. Micro Base Station 10 was designed to provide localized coverage in the form of micro cells for indoor environments and is essentially a scaled-down version of base stations used outdoors. The interior components of Base Station 10 are housed in metal cabinet 12 measuring approximately 440 mmxc3x97310 mmxc3x97488 mm (17.2 inxc3x9712.2 inxc3x9719.2 in), the separately installed antennas are not shown. A disadvantage of this base station is that its size makes unobtrusive installation difficult and inconvenient. Further, the antenna structure must be mounted separately making installation more complex and expensive. Furthermore, the heat sink required for proper operation of the internal circuit components, which may include built-in fans, is the limiting factor in reducing the size of the base station. The operation and heat removal requirements of the internal circuit components of base station 10 are described herein.
FIG. 2 shows a functional block diagram of the Micro Base Station 10 of FIG. 1. The output of transmitter TX114 is combined with transmitter TX216 with a hybrid combiner 18. The output of combiner 18 yields two components: a component 19 which is subsequently used for transmission and component 21 which is not transmitted but terminated in load resistor 24. Load resistor 24, shown separately from combiner 18 for simplicity, provides matching impedance for combiner 18 to minimize reflections for increased transmission efficiency. After emerging from combiner 18, component 19 is sent to a duplex filter 20 and then is routed to a dipole antenna 22 for transmission through the air. The component 21, after emerging from combiner 18, is dissipated as heat in load resistor 24. Roughly half of the total power emerging from combiner 18 is sent on for transmission (component 19) and the other half is dissipated in load 24 (component 21). Therefore, a signal loss of approximately a little more than 3 dB is typically experienced due to combiner 18 and load resistor 24. Similarly, transmitter TX326 and transmitter TX428 are combined in hybrid combiner 30 where the transmitted component is sent to duplex filter 32 and then to antenna 34. Similarly, the non-transmitted component from combiner 30 is terminated in load 36 and dissipated as heat. By way of example, the combination of 400 mW signal from TX1 and 400 mW from TX2 into combiner 18 results in approximately 100 mW per carrier of power transmitted from antenna 22 and 400 mW dissipated in load 24 as heat. With a comparable figure of 400 mW requiring dissipation in load 36 from TX3 and TX4, it becomes apparent that a relatively sizable heat sinking capacity capable of dissipating at least 800 mW is required for proper operation.
In view of the foregoing, it is an objective of the present invention to provide a technique for reducing the amount of heat dissipation required while maintaining substantially the same coverage area as compared to a base station with a terminated load. Further, as will be described hereinafter, the present invention provides a method and apparatus for constructing an indoor multi-carrier radio base station that is small, unobtrusive, and simple to install.
Briefly described, and in accordance with multiple embodiments thereof, the invention provides a technique for reducing heat dissipation in indoor radio base stations. In a first embodiment of the invention, a low-heat dissipating radio base station is provided comprising first and second transmitters with their output signals coupled to and combined with a hybrid combiner. The combiner generates a first output combiner signal to be transmitted through a dipole antenna, which produces vertical polarization, and a second output combiner signal transmitted through a horizontal antenna producing horizontal polarization. Prior to transmission, the output combiner signals are shifted in phase by 90xc2x0 with respect to each other by the combiner. The resulting transmission of the perpendicular oriented signals produces a substantially circular polarized field in the area of coverage. Alternatively, an elliptically polarized field may be produced by varying the magnitude and/or phase of the emitted signals.
In a method aspect of the present invention, a method of reducing the power dissipated, and subsequently the size, of a radio base station is disclosed. The method includes combining a pair of transmitter output signals with a hybrid combiner. The combiner generates a first combiner output signal and a second combiner output signal. A phase shift of 90xc2x0 is introduced by the combiner between the output signals. The combiner output signals are arranged to be emitted from an antenna such that the orientation of the signals are perpendicularly oriented to form a substantially circular polarized field. The transmission of the circular polarized field eliminates the need for signal termination in a heat dissipating load thereby reducing heat dissipation in the base station.
The embodiments of the present invention provide an efficient low-power consuming unitary radio base station in a small, convenient package. The small package design facilitates simpler mounting for unobtrusive, aesthetically pleasing installation. Further, the circular polarized field provides improved reception at the receiving station in the field of coverage. These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings.